Current orthotic design is largely unsystematic. Arbitrary applications of two-dimensional anatomical measurements have been shown to be unreliable, whatever their theoretical basis. A record of the foot contour alone is sometimes considered sufficient as the basis for orthotic design, but unless practitioners can take into account the balance of forces required for efficient locomotion, (i.e. kinetic data) they cannot accurately correct the condition of a patient.
Traditionally, orthotic manufacture takes place in two stages. The first stage, in the hands of the practitioner, is typically based on taking an impression so as to capture the foot shape, usually as a cast or model. The forefoot to rear-foot alignment in or around neutral position of the sub-talar joint is then assessed. The recorded foot shape is then modified in order to produce an improved (corrected) forefoot to rear-foot relationship. Following further clinical observations, other modifications to the cast can be made. For example the contour of the heel area and the shape of the medial and lateral longitudinal arches can be changed.
In the second stage of the process the orthotic manufacturer depends not only on the accuracy and adequacy of the prescription, but also on how accurately it can be translated for manufacture allowing for any inherent limitations of the method.
The most common problems with this process overall, whether the original design data are computer-generated or obtained manually, is the lack of consistency in both (i) how practitioners complete their evaluation and how orthotic manufacturers transform the data provided into a material orthotic product. This has led to a multitude of different approaches and a multitude of different outcomes.
Improving Current Practice the Kinetic Orthotic
Improving the efficiency of an individual's gait cycle should produce a lasting improvement in mobility and a decrease in any pathology related to biomechanical inefficiency. The design objective has been to create a “kinetic orthotic” by which muscular energy is most efficiently transferred in the gait cycle of a patient rather than an orthotic which merely compensates for abnormal foot topology, which is the functional limit of many designs.
Functioning Foot
In the functioning foot there are specific relationships between the anatomical structures commonly identified from both the frontal plane and the sagittal plane of reference. Instability can result from a misalignment between the forefoot and rear-foot which prevents the foot from functioning in a fully integrated manner. However such a simple structural (kinematic) classification as this overlooks the critical matter of how muscular energy is transmitted through anatomical structures in such a way as to confer normal motion (kinetic function) on the foot. For example, the pronation force about the sub-talor joint axis is known to increase as a result of structural misalignment. But an analysis in kinetic terms would account for the origin and magnitude of the pronation force and why this force affects the sub-talor joint. Once the problem is presented in kinetic terms, the anatomical structures are seen to play their part in the resolution and transmission of forces rather than suggesting their source.
Kinetic Processes in the Foot
Kinetic processes in the foot have been described by Kirby in terms of a dynamic equilibrium between the sum of pronation and supination forces occurring about the sub-talar joint axis. (“Rotational Equilibrium” theory (Kirby, K. A. 2001 “Sub-talar joint axis location and rotational equilibrium theory of foot function” JAPMA 91(9): 465-487)). Assessed from the sagittal plane of reference, the foot has been described as a compound pivot made up of three key pivots. The three key sagittal plane pivots can be named the “Heel rocker” the “Ankle Rocker” and the “Forefoot Rocker”. Foot pronation results when a restriction occurs at either the ankle pivot or the forefoot pivot during gait. Restriction is revealed by the inability of the ankle or forefoot rocker to function normally. Restriction can be anatomical or physiological in origin and its extent can be influenced by footwear or orthotics or both. If restriction at a key pivot sites persists of foot becomes chronically unstable, pronation becomes endemic. This process can lead to deterioration in pivotal function and further instability.
In summary, pronation is frequently observed to follow restriction at a key pivot site (a structural feature). It is now understood that it is the changing distribution of mechanical forces as a result of the restriction and/or instability that actually brings about the condition of pronation.